Use of Special Purpose Entities and Variable Interest Entities (Loan Securitization Transaction) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020
Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Aggregate unpaid principal balance of residential whole loans sold	$ 3,047,295	$ 3,047,295	$ 2,232,561
Outstanding amount of Senior Bonds, at carrying value	1,514,900	1,514,900	645,027
Outstanding amount of Senior Bonds, at fair value	530,829	530,829	869,482
Outstanding amount of Senior Bonds, total	$ 2,045,729	$ 2,045,729	$ 1,514,509
Weighted average interest rate	1.55%	1.55%	2.11%
Weighted average contractual maturity of Senior Bonds		41 years	41 years
Cash received		$ 2,788,354	$ 1,853,408
Debt issuance cost	$ 7,700	7,700	3,200
Senior notes
Variable Interest Entity [Line Items]
Cash received	277,300	1,500,000
Senior notes | Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Principal amount of Securitized debt	2,788,383	2,788,383	1,862,068
Proceeds from Senior bond sold with Step up feature		$ 387,600	568,700
Debt instrument, coupon step-up period		36 months
Senior notes | Asset-backed Securities, Securitized Loans and Receivables | Minimum
Variable Interest Entity [Line Items]
Debt instrument, basis spread on variable rate		1.00%
Senior notes | Asset-backed Securities, Securitized Loans and Receivables | Maximum
Variable Interest Entity [Line Items]
Debt instrument, basis spread on variable rate		3.00%
Senior Support Certificates | Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Principal amount of Securitized debt	$ 227,167	$ 227,167	$ 268,548